Keyed automatic gain control



Sept. 2, 1952 J. AvlNs KEYED AUTOMATIC GAIN CONTROL Filed Dec. 29, 1948 l lNvEN-rqR J2 -AVlH-s' dim/ff Y v ATTORNEY received signal strength.

Patented Sept. 2, 1952 KEYED AUTOMATIC GAIN CONTROL .lack Avins, Staten Island, N. Y. assigner to Radio v Corporation of America, acorporation of Dela Y Ware Application December 29, *1948. serial No. 617,993

The present inventionirelates to a keyed form of signal amplitude responsive system foruse in electrical circuits, and it-isconcerned more di-V rectly with a keyed signal amplitude responsive system having extraordinarilyrrapid response to both increase and decrease in applied signal amplitude.

The present invention deals more particularly, although not necessarily limitedtheretofwith a novel form of fast-acting automatic gain control system finding ready application to television receivers. Whereas-inthe prior art, there have been numerous formslof keyed automatic gain control circuits, Vthere has usually appeared some limitation as to the speed with which the circuit responds to both increases anddecre'ases of signal strengths. This limitation generally comes about by Way of the conventional-time constant networks employed in the circuits associated with development ofthe automatic .gaincontrol potential. In television systems particularly, the speed of circuit response has been at times restricted to a value which requires at least several lineY scansion intervals in the television system in which to fully correct for any change in the .The present invention provides a signal amplitucie detection systemwhichresponds so rapidly to changes -in'applied signalthat necessary cor'- rectionsV in television receiver gain due to changes in signal strength may be accomplished, With ease, before each successive line jof the reproduced television image.'

The high speedsignal amplitude responsive syst'emof' thef'pre'sent invention in'one'of its exemplary embodiments contemplates the use of a first and second electronic discharge tube connecte'd in series across a source of keying pulses. Keying pulses are also applied to the grid of the second discharge tube'so that in effect the second discharge tube Vacts as a keyed cathode .resistance for the first discharge tube. A capacitor is in turn placed in shunt with .the secondfdischarge tube. ffTh'e`v signal whose amplitude is tol be sampled is then .applied to the grid of the Vfirst discharge tube such that during the intervals in which the system is keyed, the shunt capacitor Will,= through well-known cathode follower action,-

charge toa value`substantially vequal to the instantaneous grid Qyvoltage applied to therst discharge tube. Since the discharge tubes are normally non-conductive and rendered conductive only during the Yintervals corresponding to the keying pulses;` thek voltage developed across the 2Y v A capacitor `resistance will vary only in response to changes in applied signalamplitude. V

, It is commonly known that automatic gain con--` trol circuits for use vin televisionreceiving equipment'` differ greatly from the more frequently encountered automatic gain. control circuit' embodied in receivers ,for soundv broadcast signals. Inlthe instance of the usual broadcast receiver designed for the reception oframplitude modulated carriers, it is deemed adequate that the automatic gain control potential be .produced by electrical information'gleaned from the average carrierA intensity .of the yreceived radiov signal. Clearly, such an automatic gain control circuit would not be satisfactory forcontrolling the gain of television receiver video channels as the average signal strength .of the radio frequency carrier is a function of the average image' or picture brilliance sometimes 'referred to vas f background level. As is well-known tolthose vskilled in the artto which this invention appertains, develop'-v y, gain to be changed not only in accordance with the signal A intensityvariations "of f the received radio carrier due tol undesirable fading or'other atmosphericalphenomena, but also in accordance with average picture brilliance of the imagebeing transmitted. i @Radio transmitted negative-modulated televisionV signals normally include blankingvpulses or Ablack level information which data is transmitted between each image line infcombina-Q tion with thelinesynchronizing pulse." This fline sync pulse is most commonly ysuperimposed upon the black levelsignal and these data are transmitted at some respectively predetermined constant but different carrier levels. In common television practice the 'sync pulse is transmitted at maximum carrier intensity, or'100 per cent carrier amplitude, while'the black ilevel or blank--` ing pulse is'transmitted at approximately 75'per cent of the full carrieramplitude. Theblanking impulse or blackilevel signal ini. accordance'withY Sync impulses when superimposed upon' and blanking impulse. The front porch interval is approximately 2 per cent of the line interval and represents the time between the leading edge of each black level signal and the leading edge of the line sync pulse, whereas, the back porch interval of approximately 6 percent of a line interval, is equivalent to the time interval between the end of the line sync pulse and the termination of the black level or blankout signal.

The amplitude of the radio frequency carrier as previously brought out is held constant during the transmission of all blanking and sync impulse information. During the transmission of image intelligence of the television signal, that is, during each line interval between successive blanking signals, the average amplitude of the transmitted radio frequency carrier is a function of the average light contained in the television image. Accordingly, if the reproduced image is to be predominantly dark, the average carrier amplitude will necessarily be greater than would be the case if the background levelof the image were considerably lighter, such action of course is true only in the negative system of transmission wherein white picture information is transmitted at a lower carrier level than black level information. Itis expedient, in order that the control of the gain of the receiver be in accordance with the proper aspects of the carrier, that the automatic gain control potential be developed such that its magnitude is a function of the intensity of the received carrier of the television signal during the blanking or sync intervals only, and as hereinbefore brought out, not during the transmission of the picture or line information.

Since the blanking, and more particularly the sync signals, are transmitted at greater radio frequency intensity than image line information, itV has been the general practice in television receivers to utilize some form of a peak rectifier which responds to those peak pulses of energy represented by the blanking and synchronizing signalsduring the synchronizing intervals. An automatic gain control system of this type is satisfactory to a degree so long as extraneous signals are not received in suiicient intensity to cause the peak rectifier to respond to this undesirable signal energy. In such arrangement if there is considerable noise picked up at the receiver the noise signal, as such, especially if it has an amplitude which produces total carrier signal excursions in excess of the received carrier during the synchronizing intervals, will cause the peak detector to produce an abnormal increase in rectifier energy andV therefore produce abnormal increase in automatic gain control potential which results in a generally undesirable reduction of the receiver sensitivity as a result of such noise. This, of course, interferes with the proper operation of the receiver and produces fluctuations of the reproduced image brightness with possible periods following the noise bursts during which inadequate synchronizing information is applied to the synchronizing circuits due to an abnormally high reduction in receiver gain. This latter effect may tend to produce tearing out or other destructive disturbances in the reproduced image. Furthermore, in such systems the automatic gain control potential that is developed by each successive sync or blanking constant throughout one or more succeeding field intervals regardless of changes in signal strength. Due to the operation of the peak detector and the presence of this time delay circuit, such a circuit may also respond rapidly to high intensity noise pulses but not allow the receiver to recover as quickly from the effects of noise as kit responded to the noise with the result that noise.

If the high-speed signal amplitude responsive system of the present invention is keyed by a signal held in synchronism with the line scansion function ofthe television receiver and the received television signal is applied for sampling by the system so that the voltage developed across the shunt capacitor may be used as an AGC control voltage, `the above related disad- Vantages of prior art AGC systems may be overcome.

It is therefore a purpose of the present invention to provide a keyed electronic circuit having extraordinarily high-operational speed which will develop a voltage in accordance with the amplitude of an applied signal during intervals corresponding to keyed state of t-he circuit.

It is another purpose of the present invention to provide an improvedgautomatic gain control circuit simple Vin operation for embodiment in radio receivers adapted to translate standard types of television signals.

Another purpose of the present invention resides in the provision of an improved automatic gain control circuit for a television receiver, wherein the circuit is rendered unresponsive to received signals during intervals between successive blanking periods.

lIt is another purpose ofthe present invention to provide a simple, economical andhigh speed system for radio receiver keyed automatic gain control operation which requires avminimum of apparatus ,and exhibits substantially the same high speed of response to decreases and increases in received signal strength. A

The novel features which are believed to be characteristic of the present invention are set forth in the appended claims. The invention itself, however, both to its organization and method of operation, will be best understood from the teachings of the following description especially when taken in connection with the accompanying drawings wherein:

The figure is one embodiment of the present invention as applied toa conventional television receiving circuit.

Referring now to the figure, there is indicated in block form I0 certain well-known components ofAv a conventional television receiver including an R.V F. amplifier, an oscillator, a first detector or mixer, an intermediate frequency amplifier, a video demodulatcr, and a D. C.connected video amplifier. Examples of typical arrangements ap'- plicable to the functions depicted by block I0 as well as other block representations in the drawings hereinafter to be described are given in an article entitled Television Receivers by Antony Wright appearing in the March 1947 issue Yof RCA Review.. The signals are picked up by antenna. I2 and through the medium of' transmission line .I4 areapplied to the input of the receiver as shown. After the received signal has been ydemodulatedit is amplified bythe video 'ampliiiergiboth the second detector andspicture D. `C. component being retained. The demodulatedlvideo signal -I6- is then appliedv to the cathode 24 of `arr-image -reproducingcathode raytube 20;. Airesistor 22 is connected betweenthe grid 18; and vground potential such that the rheostat 26..connected.with a source of negative potential28fmay -be varied to provide a conventional brightness control action on the reproduced-image;

The video signal I6 is also applied to the infV put of the synchronizing 'signalv separatoriy 30 which eiects'separation ofthe vertical and. Syn-p chronizing .pulses and respectively applies. them to. 'the vertical; f and horizontal deflectionxdrive. generators 32 and` 34. The output of the lvertical deiieetion drive generator is thenconventionallyapplied to the vertical deflection outputstage 36. having output terminalsv YTYffor connection with the vertical deiiectionwinding Y--Y of the deflection yoke 38v associated withv the reproducing 'Atube 20. Accordingly, the voutputofhorizontal deflection drive.- generator 34 is applied to the control grid 40 of the horizontaldeflection output Atube 42.l The .anode44 of the .vacuum tube 42 is supplied withbiasingpotential through the output transformer .winding46 from a B+. power supply terminal 48.. lThezcathode 50 of theA horizontal output tube.. ,42. is connected lwith, ground through. cathode biasingresistor-,52 in turn shunted by. by-pass. capacitor 54. The

screen grid 56 is indicated as being conventionally supplied with a suitable positive potential. rThe secondary .58 of theghorizontal output transformer is as adaptedfonconnectionat X-X withthe terminals. X+,X-of the lhorizontal deiiection winding ofthe yoke 38. `-Asuitable damping circuit, V60 is. placed across. the yoke winding to insure properwaveform of the current variations, in the horizontal deflection winding X-X. An `auxiliary secondarywinding -|2y with one. terminal .connected Withground is .also shown on the horizontal output .transformer-. to. provide a source of positively. extending-.pulses such as shown at 64. During propersynchronous operation ofthe television receiver. these pulses corresponding to the return trace of the horizontal deflection-cycle will also be in synchronism with the received synchronizingpulsesdl6A OfthSSgIlals. i

According Vto the presentinvention, the signal I6 is also applied to the grid 66 `of the vacuum tube 68--which has its cathode'10 connectedwith ground potential through capacitor 12. This capacitor 1-2 is shunted by'vacuum tube 14 having its control elecrode 16v supplied with the positively vextending keying pulses' '64 through the coupling medium of y capacitor 118. It will be noticedthat these'pulses 64' are also applied'to the anode 80 of vacuum tubev68v.

Inthe operation of the present invention. the pulss'.64 are of suicient amplitude to cause the "normally non-conductive discharge 'tubesv 68 and`14 tobecome' conductive during the blanking period; It will be seenv that Aduring the conduction of both vacuum tubes 68 and 14 that the vacuum tube 14 effectively functions as a cathode resistance for the vacuum tube 68. As will be appreciated to those familiar with well known cathode follower action of electronic discharge tubes, during the period of conduction of 6 both: series :discharge tubes,Y f the .cathode 10 of vacuum tube. 68 will-.assumea potential `correspending-totheA potential instantaneously applied to the'controlgrid Bti-at the iti-me .of conduction; 1

If-the capacitori12 is madesufilciently smallin ,consideration of ,the conductor impedance' lofvacuum'tube '6 8-, it will be seen that the capacitor Willvbe virtually. fully charged, ltduring the con'- duction` timecorresponding to the pulse du'ration to the potential. value attained byA cathode 10i.A .'Thus,`it will bei seen that' the voltage appearing across capacitor 12 during lthe conduction' vtime' willH correspond tothe amplitude of thefreceivedisignal during this period of keyed conduction.` .Immediately following this keyed period bothtubes l|8 and 10 are` renderedA nonconductivep! Hence, for lack of a substantial discharge fpath" the voltage ofthe capacitor i12.

will .maintain'a virtuallyconstant value until the next keyed interval lat' which time its terminal potential will againbe altered'in response tothe actualsignalfamplitude applied` to the control electrode l|36 which in. turn represents received j The anode 8810i the 'vacuum tube v84 is then connected vthrough load resistor to a source'of positive potential 92 lso that the positive voltage at the terminal 94 will vary inversely withL the received signal strength. 'The positive voltage at terminal 94 may then be rtransduced into a Anegative voltage suitable for application to the grids ofthe R.'F. and I. F. amplifier stages in block |-0 by a number of conventional circuit arrangements. The one shown here employs resistors 96 and 98 connectedfrom :anode 88 to a source of negative `potential |00.e The values of resistors 96 and 98 arel shownwproportionedv to permit the negative voltageappearing atitheir juncture" |02 to be; suitable for application as a negative AGC potential. iAs shown,A thev D. C. amplifier 84 is provided-'with suitable operating bias by means of .cathode vresistor |04 .in connection with a series .resistor |06 :connected with a positivev potential source |08. 'Ihe positive voltage de" veloped across resistor. |04 as a result of currents applied throughresistor |06 may be such to bias the amplifier tube 84 to cut-oi under no-signal operating conditions.'- In this way, the tube Y84 may `be made .to supply the necessary threshold characteristic for the`AGC system. It is clear that this threshold characteristic mayl also be achievedby 'suitable biasing of the vacuum tube es if'desied. 'l

yAs-fan example of AGCv correction produced by the system `shown inthe lfigure consideration will be given to circuit conditions obtaining from an increase in signal strength. It will be assumed that the received signal I6 is of suificient amplitude to cause development across capacitor 12 of a voltage in turn suiiiciently positive to cause conduction and linear operation of the D. C. amplifier 84. An increase in signal strength will then cause the instantaneous potential of the control grid 66 of vacuum tube 68 to in turn be ascensoY more positive duringv the. action of keying pulse 64,.in1keying into. conduction both vacuum: tubes 66 and '713;.3 .Through cathode: followeriaction capacitorplZ WillL then undergo an :increase in positive terminal potential. .The potential on grid821of D. CJ. amplier 84 will also become more positivev and produce` a corresponding increase in` the ;'plate current' through load i resistor '90 which in turn willproduce a drop in the positive potentialuappearing at terminal 94. Since .the potential-at lll2 is'already negative through the dropping action-of the bleeder resistors 96sand 98, fthevoltage applied to the AGC terminal H will become, more vnegative to decrease thegain of receiver L0. andtend to compensate for the increasein'the receivedsignal strength.v Thereverseiaction :of course would accompany adeoieasein signal strength. Aydecrease in signal strengthfwill establish the grid 66 of vacuum tube wat amore negative potential during keyed conductionof tube and 14 thus causing the capacitorp12 to undergo a drop in positive terminal voltage. This, in turn, produces a negative swing on the grid 82 of the D. C. amplierM/andcauses the potential appearing at terminali 102 tobecome more positive, This increases the gain of the receiver in'van eifortjto compensate for the/,decreasein signal strength. y i As -hereinbefore brought out, the.'Y inherently rapid action of this signal amplitudedetection systerncomprising-tube'-68 and 14; which is the Subj ect 'of the present invention, permits api-actically instantaneous` change in applied AGC terminal potentialat :the end of every :line scansion interval corresponding in time to the occurrence of the keying pulses 64. Thus, thezfAGC system may easilycorrect 'for changes in received signal intensityon a line for `line basis in thereproduced television image... n

1 Another feature of thef present invention ree,V sides-in theiact that,the-:AGC detection system requires virtuallygno' power from the video signal Source but derives its 1 energy for developing. the correcting potential from .the horizontal deflection outputstage.. In this connection replacement of tube. ESfby a pentodev/illr provide' more immunity-to any changes-iin the amplitude of'pu'lse 54.` This latter' consideration may. befof particulari importance WhereV the horizontal-l deflection output vstage is. adapted to function asa; pulse stepup hi'ghvoltage power supply forthe acceleratinganodefo'f the-cathode ray reproducing-Stube 20. Under suchconditions the pulseil may vary in'amplitude' in accordance vvvith. catliodezray beam current vari-ations v'acc'oz'npanying changes in D. C. .picture background level-.t 'On-ithe other handunder circuit conditions wherein Ythe .pulse amplitude rremains virtually constant in-.amplitude, it is apparent then that the duo-triade '14v and-,Gamay be combined into a single envelopel to provide certains-pace economiesin the practiceofv theV invention. v

In eXtrem'elyhigh-.speed actiorrof the circuit will automatically suggest other advantageous uses forit yother-than inAGC systems. e. For instance in systems where a certainperiodically recurrent portion cuftl'ie modulation envelope was varied in accordance with intelligence signal, it is conceivable that pulses similar to 64 could be synchronized to drive the 'tubes' 66 and. vinto conduction duringzthese periods and vthusadapting the detector circuit as a demodulatorlfor this signalintelligence. vFrornl` the foregoing; .it is seen that the-applicantv hasip'rovided` a simple, novelcompact, economical, Vand effective signal .amplitude responsivedetection system which is inherently :versa tile in itsapplication torelectronic equipment'and finds particular application inatelevision high speed AGC.system.` n.

What is claimed is: *.1

'In' a radio receiver adapted :to receive signals of the television type`,;said lsignalsfhaving are= current synchronizing pulse component.` repre- Sentinganired percentageof carrier modulation, an .automatic gaincontrol circuit for the .radio receiver comprising, -in combination, a source oftiming-'..sign'als extending `in a positivedirection' and-synchronouswith said recurrent synchronizinglpulsefoomponent, a first statically no`n=c`on dotiveelectron discharge tube having an anode, a cathode, and a control electrode, a connection. for applying said timing signals to the anodeof saidfnrstele'ctron discharge tube to rendersaid tube periodically conductive, means for applying tothe control electrode of said first electron discharge tube televisionsignals IWhose.recurrent synchronizing pulse component extends'in a pose` iti've direction, a point of reference potential, a capacitoroonneoted'between the cathode of said r'st"electron-discharge tube and 'the point of referencepotential, a second statically nonconductive electron Ydischarge tube having-an anode, a cathode, and a control electrode, a connection betvieentlie Aanode of the second electron discharge.tubeV and the'cathode of the rst electron discharge tube, a connection between the cathode ofi-'fthe second electron discharge tube and the pointfofreference potential,v a connection infcluding" a capacitor'between said source of timing'sig'nals and the-control electrode'of the'secondk electron discharge' tube to render said tube periodically conductive, Wherebythe voltage de-' veloped aoros'ssaidv capacitor is av function of tlievalue ofthe television signal when said first disohargetube is rendered conductive,- and means to v'aiav 'i;ln=igain of'said-receiver in. accordance with saidvoltage.' v j '-1 JACK AVINS.

REFERENCES GITED.

Theiiollowing 'references are of record inftlfi'e file of this patent:

"UNITED STATES PATENTSV Number Name Date 2,l90,753 Browne Yet al. Feb. 20, .1940 2,197,900 Schlesinger Apr. 23, 194.0 2,307,375 Blumlein et al. Jajn."5, 1943 2,323,885 Wills July 13, 1943 2,466,705 Hoeppner -Apr. 12, 194,9 2,467,486v- Krumhansl et a ns Apr. 19, 1949 FOREIGNl PATEN'IS4 liumb'erl Country Date .507,239 Great Britain June 6. 1939 512,109 Great Britain' Aug. 29', 1939' 845,897

, France sept. 4. 1939 

